Electrochemical measuring apparatus for determining the pH or the gas or ion concentration of biological fluids such as blood typically include a high impedance electrode device as the sensor. For example, to measure pH, sodium, potassium, or chloride, high impedance measuring electrodes in the range of 100 to 3,000 megaohms are used. These high impedance electrodes can be effected by externally generated voltages from sources such as RF antennas and from other sources of electromagnetic, static electric, and galvanic activity.
A peristaltic pump is typically used in electrochemical measuring systems to move the sample fluid past the measuring electrodes. A simple peristaltic pump consists of one or more rollers in contact with an elastomeric tubing through which the fluid flows. The peristaltic tubing is generally stretched over the rollers to pinch (close off) the tubing at one or more points of contact. Fluid entrapped by the contact points is pushed through the tubing by the action of the rollers moving along the stretched tubing.
Common elastomeric materials used in peristaltic pumps (e.g., silicone and soft PVC) can generate a static charge when a pump roller contacts and rolls and/or slides over the surface to move the fluid within the tube. Such build-up is conducted through the pumped fluid and can be detected in the high impedance measuring devices as "amplified peristaltic noise". The frequency of noise is dependent on the pump cycle and the number of rollers in the pump. The net effect is a degradation of the precision of the analytical measurements.
Previous attempts to eliminate peristaltic noise have involved electronic filtering between the measuring electrode and meter by either an analogue filter or by digital methods. Such filtering is undesirable because it dampens the entire signal coming from the electrode, both the ion measuring component and noise. Further, analogue capacitive filtering at the measuring electrodes is undesirable because the capacitance effects an increase in the response time of the electrode. Digital filtering is expensive and requires additional time to make computations.